Nitride-based compound semiconductor electron device

ABSTRACT

A GaN-based FET has a buffer layer structure including GaN first buffer layer and an AlGaN second buffer layer between a substrate and an active layer structure including a channel layer and a donor layer. The GaN first buffer layer and the AlGaN second buffer layer reduce dislocation defects in the active layer structure and allows the FET to have a lower leakage current and a satisfactory pinch-off characteristic. A plurality of GaN first buffer layers and a plurality of AlGaN second buffer layers may be deposited alternately one on another on the substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nitride-based compound semiconductorelectron device, and more particularly, to a semiconductor electrondevice, such as a field-effect transistor, including nitride-basedcompound semiconductors as min component thereof.

2. Description of the Related Art

In general, field-effect transistors (FETs) having nitride-basedcompound semiconductors, such as GaN-based compound semiconductors, aresubstantially free from a burn-up failure even at an operatingtemperature as high as close to 400° C., and thus are drawing attentionas solid laser devices operating at a higher temperature range.Hereinafter, the FETs having GaN-based compound semiconductors arereferred to as GaN-based FETs.

However, it is difficult to manufacture a single-crystal substratehaving a large diameter in the case of a GaN-based crystal differentlyfrom the cases of Si crystal, GaAs crystal, and InP crystal. Thus, it isdifficult to form a semiconductor layer structure in a GaN-based FET byepitaxially growing a crystalline layer of a GaN-based material on a GaNsingle-crystal substrate. Accordingly, in manufacturing a GaN-based FET,a crystalline layer of GaN-based semiconductor is grown by using theprocess such as described hereinafter. It is to be noted that theprocess will be described taking as an example a lateral GaN-based FETshown in the schematic drawing of FIG. 3.

First, on a single-crystal sapphire substrate 11 for the crystal growth,an intermediate layer 12 including a GaN single crystal as a maincomponent thereof is deposited by using an epitaxial crystal growthprocess, such as a MOCVD process, while appropriately selecting thefilming conditions for the crystal growth, such as selecting a growthtemperature at 500 to 600° C., for example.

Thereafter, a buffer layer 13, a channel layer 14, a donor layer 15, anda contact layer 16 are consecutively deposited on the intermediate layer12 by using a GaN epitaxial growth process. Thereafter, an electrodegroup is formed on the semiconductor layer structure, the electrodegroup including a source electrode 17 a and a drain electrode 17 c,which are coupled to the semiconductor layer structure with an ohmiccontact, and a gate electrode 17 b, which is coupled thereto with aSchottky contact or MIS (Metal-Insulator-Semiconductor) structure, tothereby achieve the lateral GaN-based FET as shown in FIG. 3.

In the conventional structure of the GaN-based FETs, as described above,the lattice constants are significantly different between the sapphiresubstrate 11 and the GaN single crystal, and hence, the intermediatelayer 12 generally has therein dislocation defects caused by the latticemismatch with the sapphire substrate 11 and extending perpendicular tothe depthwise direction of the film. The dislocation density in thesemiconductor layer structure usually has a value of about 1×10⁹ to1×10¹⁰ cm⁻². Thus, in the GaN-based FET, the semiconductor layerstructure including the channel layer 14, the donor layer 15, and thecontact layer 16 is formed on the intermediate layer 12 having suchdefects.

More specifically, in the FET having the above-described layerstructure, dislocation defects existing in the intermediate layer 12 aretransmitted directly in the depthwise direction (or vertical direction)into the semiconductor active layer structure of GaN crystal having theFET function. The number of existing dislocation defects is, forexample, about 100 per unit area of 1 μm² of the semiconductor layerstructure. Hence, the GaN crystal structure configuring thesemiconductor active layer structure is degraded in the film qualitythereof. This causes a problem of leakage current in the resultantGaN-based FETs to degrade the pinch-off characteristic.

More generally, there has been the problem, in a semiconductor electrondevice having nitride-based compound semiconductors, that leakagecurrent flows due to dislocation defects in the layer structure otherthan the channel layer, resulting in that a satisfactory pinch-offcharacteristic cannot be obtained.

Accordingly, various techniques have been examined for suppressing thedislocation defects, i.e., one of the factors that cause occurrence ofthe leakage current. Japanese Patent Laid-Open Publication No.2003-059948, for example, describes a method that suppresses dislocationdefects by providing a buffer layer structure wherein AlN layers and GaNlayers are alternately deposited one on another on a silicon substrate.However, even with this method, the leakage current cannot be reduced toa satisfactory degree, and thus a satisfactory pinch-off characteristiccannot be obtained heretofore.

SUMMARY OF THE INVENTION

In view of the above, it is an object of the present invention to solvethe above problem and provide a semiconductor electron device includingnitride-based compound semiconductors and having an excellent pinch-offcharacteristic.

The present invention provides a nitride-based compound semiconductorelectron device including: a substrate; and a semiconductor layerstructure including a buffer layer structure, a channel layer and adonor layer, that are consecutively formed in this order on thesubstrate, wherein the buffer layer structure includes: at least onefirst buffer layer comprising as a main component thereof a compoundsemiconductor expressed by the general formula ofAl_(x)In_(y)Ga_(1-x-y)As_(u)P_(v)N_(1-u-v) (where 0≦x≦1, 0≦y≦1, x+y≦1,0≦u≦1, 0≦v<1, and u+v<1); and at least one second buffer layercomprising as a main component thereof a compound semiconductorexpressed by the general formula ofAl_(a)In_(b)Ga_(1-a-b)As_(c)P_(d)N_(1-c-d) (where 0≦a≦1, 0≦b≦1, a+b≦1,0≦c<1, 0≦d<1, and c+d<1), and wherein the first buffer layer and thesecond buffer layer have different bandgap energies, and havetwo-dimensional electron gas density or densities therein not greaterthan 5×10¹² cm⁻².

In accordance with the semiconductor electron device of the presentinvention, the buffer layer structure, including two different layerswhich are different in the quality of material, suppresses the leakagecurrent caused by the two-dimensional electron gas, that is accumulatedin one of the layers having a smaller bandgap energy and being disposedadjacent to the interface between the two different layers. Thesuppression of the leakage current can provide a semiconductor electrondevice including the nitride-based compound semiconductor with anexcellent pinch-off characteristic. In particular, the advantage is moreremarkable if a larger current of greater than 1 ampere is output or ahigh power source voltage of greater than 100 volts is applied in thesemiconductor electron device, whereby it has an excellent pinch-offcharacteristic and a high withstand voltage.

Further, the semiconductor electron device of the present invention mayhave an intermediate layer between the substrate and the buffer layerstructure, and a contact layer between the donor layer and theelectrodes for reducing the contact resistance therebetween. Moreover, asubstrate made of a material such as sapphire, SiC, Si, GaAs, or GaP maybe preferably used as the substrate for crystal growth.

The above and other objects, features and advantages of the presentinvention will be more apparent from the following description,referring to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view exemplifying the layer structure of asemiconductor electron device according to a first embodiment of thepresent invention.

FIG. 2 is a cross-sectional view exemplifying the layer structure of asemiconductor electron device according to a second embodiment of thepresent invention.

FIG. 3 is a cross-sectional view of the layer structure of aconventional semiconductor electron device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the present invention will be described below in more detail by wayof preferred embodiments thereof with reference to the accompanyingdrawings, wherein similar constituent elements are designated by similarreference numerals throughout the drawings.

Referring to FIG. 1, a semiconductor electron device, generallydesignated by numeral 100, according to a first embodiment of thepresent invention includes: a silicon substrate (semiconductorsubstrate) 11; a layer structure including a GaN intermediate layer 12,a pair of buffer layers including a first buffer layer 13 a and a secondbuffer layer 13 b, an GaN channel layer 14, an AlGaN donor layer 15, anda GaN contact layer, which are consecutively deposited on the siliconsubstrate 11; and an electrode group including an Al/Ti/Au sourceelectrode 17 a formed on the contact layer 16, a Pt/Au gate electrode 17b formed on the donor layer 15, and an Al/Ti/Au drain electrode 17 cformed on the contact layer 16.

In the above structure, the first buffer layer 13 a is made of GaN, andthe second buffer layer 13 b is made of AlGaN. The AlGaN second bufferlayer 13 b has a bandgap energy larger than the bandgap energy of theGaN first buffer layer 13 a. The contact layer 16 formed on the donorlayer 15 has a recess in which the Pt/Al gate electrode 17 b directlycontacts with the donor layer 15, whereas the Al/Ti/Au source and drainelectrodes 17 a and 17 c are coupled to the donor layer 15 with anintervention of the contact layer 16. The contact layer 16 is providedfor reducing the resistance of the ohmic contact between the source anddrain electrodes and the donor layer 15.

A process used for manufacturing a sample of the semiconductor electrondevice shown in FIG. 1 will be described hereinafter.

A MOCVD apparatus was used as a growth apparatus, and a siliconsubstrate chemically etched with hydrofluoric acid was used as thesilicon substrate 11.

The silicon substrate 11 was introduced into the chamber of the MOCVDapparatus, and the chamber was evacuated with a turbo pump until thedegree of vacuum therein became 1×10⁻⁶ hPa or less, followed by heatingthe substrate 11 up to a temperature of 800° C. at a vacuum degree of100 hPa. After the substrate temperature became stable, trimethylgallium (TMG) and NH₃ were introduced as source materials onto thesurface of the silicon substrate 11 at flow rates of 58 μmol/min. and 12l/min., respectively, with the silicon substrate 11 being rotated at 900rpm, to thereby grow thereon the GaN intermediate layer 12. The growthtime was 4 minutes and the film thickness of the intermediate layer 12was about 50 nm.

Subsequently, the substrate temperature was raised up to 1030° C.,followed by introducing trimethyl gallium (TMG), NH₃, andbis-cyclopentadienyl magnesium (CP₂Mg) onto the intermediate layer 12,at flow rates of 58 μmol/min., 12 l/min., and 0.01 μmol/min.,respectively, to grow the GaN first buffer layer 13 a. The growth timewas 100 seconds and the film thickness of the GaN first buffer layer 3 awas 50 nm. It is to be noted that Mg was added therein in an amount of1×10¹⁸ cm⁻³.

Thereafter, trimethyl gallium (TMG), trimethyl aluminum (TMA), NH₃, andbis-cyclopentadienyl magnesium (CP₂Mg) were introduced onto the GaNfirst buffer layer 13 a, at flow rates of 29 μmol/min., 29 μmol/min., 12l/min, and 0.01 μmol/min., respectively, to grow the Al_(0.5)Ga_(0.5)Nsecond buffer layer 13 b. The growth time was 40 seconds and the filmthickness of the Al_(0.5)Ga_(0.5)N second buffer layer 3 b was 20 nm. Itis to be noted Mg was added therein in an amount of 1×10 ¹⁸ cm⁻³.

By inserting the two buffer layers 13 a and 13 b having differentcompositions between the silicon substrate 11 and the intermediate layer12, as described above, the dislocation was deflected in thetransmission direction thereof from the depthwise direction or growthdirection, to thereby reduce the dislocation defects in the active layerstructure overlying the buffer layers 13 a and 13 b.

In this manner, the amount of dislocation defects was suppressed down toabout 1×10⁸ cm⁻², whereby an AlGaN/GaN heterostructure having reduceddislocation defects was obtained.

Subsequently, trimethyl gallium (TMG) and NH₃ were introduced onto thebuffer layer 13 at flow rates of 58 μmol/min. and 12 l/min.,respectively, to thereby grow the GaN channel layer 14. The growth timewas 1000 seconds and the film thickness of the channel layer 14 was 500nm.

Thereafer, trimethyl gallium (TMG), trimethyl aluminum (TMA), and NH₃were introduced at flow rates of 41 μmol/min., 17 μmol/min., and 12l/min. respectively, to thereby grow the AlGaN donor layer 15. Thegrowth time was 40 seconds and the film thickness of the donor layer 15was 20 nm.

Further, trimethyl gallium (TMG), SiH₄, and NH₃ were introduced at flowrates of 58 μmol/min., 0.01 μmol/min., and 12 l/min., respectively, tothereby grow the GaN contact layer 16 on the donor layer 15. The growthtime of the contact layer 16 was 40 seconds and the film thickness ofthe contact layer 16 was 20 nm. Subsequently, the contact layer 16 wassubjected to etching to form the recess therein, followed by depositingthe source electrode 17 a, the gate electrode 17 b, and the drainelectrode 17 c on the contact layer 16 and the donor layer 16, by anevaporation technique.

While applying a voltage between the source electrode 17 a and the drainelectrode 17 c and a reverse voltage to the gate electrode 17 b in theresultant GaN-based FET, the pinch-off characteristic of the FET wasexamined. It was found that the pinch-off of the FET occurred when thevoltage applied to the gate electrode 17 b was −3 volts. The withstandvoltage between the source electrode 17 a and the drain electrode 17 cduring an off-state of the FET was 523 volts.

Next, a test sample was fabricated for measuring the leakage current ofthe FET. The contact layer 16, the donor layer 15, and the channel layer14 were removed by etching, and two ohmic electrodes were formed on thebuffer layer 13. While applying a voltage between the source electrode17 a and the drain electrode 17 c, leakage current was measured, whichexhibited 0.1 μA. This value is about {fraction (1/1000)} of the leakagecurrent of 100 μA of typical semiconductor electron devices manufacturedby using a conventional technique.

Referring to FIG. 2, a semiconductor electron device, generallydesignated by numeral 200, according to a second embodiment of thepresent invention has a buffer layer structure 13A including 30 GaNfirst buffer layers and 30 AlGaN second buffer layers which arealternately deposited one on another on the intermediate layer 12.

More specifically, the semiconductor electron device 200 includes thesilicon substrate 11, the intermediate layer 12, the channel layer 14,the donor layer 15, and the contact layer 16, which are deposited andconfigured similarly to those of the first embodiment with the exceptionof the buffer layer structure 13A.

In the buffer layer structure 13A, the GaN first buffer layers aredesignated herein by numerals 13 a _(n) (13 a ₁, 13 a ₂ . . . 13 a ₃₀),as viewed consecutively from the bottom, whereas AlGaN second bufferlayers are designed herein by numerals 13 b _(n) (13 b ₁, 13 b ₂ . . .13 b ₃₀), as viewed consecutively from the bottom. It is to be notedthat the AlGaN second buffer layers 3 b _(n) have a bandgap energylarger than the bandgap energy of the GaN first buffer layers 13 a _(n).

In the second embodiment, the source electrode 27 a and the drainelectrode 27 c are made of Ta silicide which is in contact with thecontact layer 16, whereas the gate electrode 17 b is made of Pt/Au whichis in direct contact with the donor layer 15.

The GaN first buffer layers 13 a ₁, 13 a ₂ . . . 13 a ₃₀ need not havethe same bandgap energy, so long as the bandgap energy of each of theGaN first buffer layers 13 a ₁, 13 a ₂ . . . 13 a ₃₀ is smaller than thebandgap energy of each of the AlGaN second buffer layers 13 b ₁, 13 b ₂. . . 13 b ₃₀.

Likewise, the AlGaN second buffer layers 13 b ₁, 13 b ₂ . . . 13 b ₃₀need not have the same bandgap energy, so long as the bandgap energy ofeach of the AlGaN second buffer layers 13 b ₁, 13 b ₂ . . . 13 b ₃₀ islarger than the bandgap energy of each of the GaN first buffer layers 13a ₁, 13 a ₂ . . . 13 a ₃₀.

A process used for manufacturing a sample of the semiconductor electrondevice of the second embodiment will be described hereinafter byspecifically reciting the differences between the present embodiment andthe first embodiment.

After depositing the intermediate layer 12, the substrate temperaturewas raised up to 1030° C. Trimethyl gallium (TMG), NH₃, andbis-cyclopentadienyl magnesium (CP₂Mg) were introduced onto theintermediate layer 12 at flow rates of 58 μmol/min., 12 l/min., and 0.01μmol/min., respectively, to thereby grow the GaN first buffer layer 13 a₁. The growth time was 20 seconds and the film thickness of the GaNlayer 13 a ₁ was 10 nm. It is to be noted that Mg was added therein inan amount of 1×10¹⁸ cm⁻³.

Subsequently, trimethyl gallium (TMG), trimethyl aluminum (TMA), NH₃,and bis-cyclopentadienyl magnesium (CP₂Mg) were introduced at flow ratesof 29 μmol/min., 29 μmol/min., 12 l/min., and 0.01 μmol/min.,respectively, to thereby grow the Al_(0.5)Ga_(0.5)N second buffer layer13 b ₁. The growth time was 20 seconds and the film thickness of theAl_(0.5)Ga_(0.5)N second buffer layer 13 b ₁ was 10 nm. Mg was addedtherein in an amount of 1×10¹⁸ cm⁻³.

The GaN first buffer layers 13 a _(n) and the Al_(0.5)Ga_(0.5)N secondbuffer layers 13 b _(n) were alternately grown one on another in theorder of 13 a ₁, 13 b ₁, 13 a ₂, 13 b ₂ . . . 13 a ₃₀, 13 b ₃₀, tothereby form 30 layers each.

The processes used for fabricating the channel layer 14, the donor layer15, the contact layer 16, and the electrodes 17 a, 17 b, 17 c on thesecond buffer layer 3 b ₃₀ are similar to the those in the firstembodiment.

The pinch-off characteristic was evaluated in the second embodiment asin the first embodiment. The pinch-off occurred in the FET when thevoltage applied to the gate electrode 17 b was −3 volt. The withstandvoltage between the source electrode 17 a and the drain electrode 17 cduring the off-state was 648 volts.

In addition, the leakage current of the FET decreased down to about 5nA. This value is about {fraction (1/20)} of the leakage current of thesemiconductor electron device 100 manufactured in the first embodiment.

It is generally desired that the buffer layer 13 be electrically neutralin order to reduce the leakage current of the FET. However, residualimpurities in the GaN-based compound semiconductor generally exist in anamount of about 1×10¹⁶ cm⁻³ and at least in an amount of about 5×10¹⁵cm⁻³, thereby causing the buffer layer 13 to assume the n-typeconductivity.

The concentration of p-type impurities to be added in order tocompensate for the n-type carriers of the buffer layer structure amountsto at least about 1×10¹⁶ cm⁻³. However, since the activation rate of thep-type impurities is relatively low compared to the n-type impurities,the p-type impurities need to be added in an amount of about 1×10¹⁸cm⁻³. Hence, in the present embodiment, Mg was added as p-typeimpurities in an amount of 1×10¹⁸ cm⁻³. It is to be noted that if theamount of p-type impurities to be added for the compensation exceeds1×10²¹ cm⁻³, the buffer layer 13 may assume the p-type conductivity,which is undesirable. Thus, the amount of p-type impurities should bepreferably 1×10²¹ cm⁻³ or less.

Next, a semiconductor electron device according to a third embodiment ofthe present invention will be described. The third embodiment is similarto the first embodiment in the basic structure thereof, and is differentfrom the first embodiment in that the first and second buffer layershave different thicknesses by using different growth times for the firstand second buffer layers.

Table 1 shows the leakage current measured in the third embodiment whilechanging the film thicknesses, and the two-dimensional electron gasdensity measured in a CV (capacitance-voltage) measurement, similarly tothe measurements in the first embodiment. TABLE 1 Thickness First 0.20.5 1.0 5.0 10 20 25 30 (nm) Buffer Layer Second 0.2 0.5 1.0 5.0 10 2025 30 Buffer Layer Leakage Current (μA) 132 10.0 0.1 0.1 0.1 1.0 54 97Two-Dimensional 0.1 1.0 1.0 2.0 4.0 5.0 12 23 Electron Gas Density (10¹²cm⁻²)

Next, a fourth embodiment of the present invention will be described.The fourth embodiment is similar to the first embodiment in the basicstructure thereof, and is different therefrom in that the Al compositionof the second buffer layer is changed by changing the amount oftrimethyl aluminum (TMA) introduced during the growth of the secondbuffer layer.

Table 2 shows the leakage current measured in the fourth embodimentwhile changing the film thicknesses, similarly to the measurements inthe first embodiment. TABLE 2 Al Composition of Second Buffer 0.1 0.30.5 0.7 0.9 Layer Leakage Current (μA) 10 5.0 0.1 0.5 5.0

It is to be noted that the present invention is not limited to the aboveembodiments. For example, although the thicknesses of the first andsecond buffer layers are equal in the above embodiments, the thicknessesof the first and second buffer layers may be different. In addition,although the first buffer layer and the second buffer layer are made ofGaN and AlGaN, respectively, in the above embodiments, the first bufferlayer and the second buffer layer may be made of InGaN and AlGaN,respectively, or may be made of InGaN and GaN, respectively.Furthermore, although the gate electrode is made of Pt/Au in the aboveembodiments, the gate electrode may be made of Pd, W, Ni etc. alone orin combination thereof.

By using the present invention, the leakage current of semiconductorelectron devices can be suppressed, and the withstand voltage thereofcan be improved. In particular, it is possible to manufacturesemiconductor electron devices that operate with a higher withstandvoltage and a lower ON-resistance and are excellent in the pinch-offcharacteristic thereof.

1. A nitride-based compound semiconductor electron device comprising: asubstrate; and a semiconductor layer structure including a buffer layerstructure, a channel layer and a donor layer, that are consecutivelyformed in this order on said substrate, wherein said buffer layerstructure includes: at least one first buffer layer comprising as a maincomponent thereof a compound semiconductor expressed by the generalformula of Al_(x)In_(y)Ga_(1-x-y)As_(u)P_(v)N_(1-u-v) (where 0≦x≦1,0≦y≦1, x+y≦1, 0≦u<1, 0≦v<1, and u+v<1); and at least one second bufferlayer comprising as a main component thereof a compound semiconductorexpressed by the general formula ofAl_(a)In_(b)Ga_(1-a-b)As_(c)P_(d)N_(1-c-d) (where 0≦a≦1, 0≦b≦1, a+b≦1,0≦c<1, 0≦d<1, and c+d<1), and wherein said first buffer layer and saidsecond buffer layer have different bandgap energies, and havetwo-dimensional electron gas density or densities therein not greaterthan 5×10¹² cm⁻².
 2. The semiconductor electron device according toclaim 1, wherein said first buffer layer has a thickness of not lessthan 0.5 nm and not greater than 20 nm, and said second buffer layer hasa thickness of not less than 0.5 nm and not greater than 20 nm.
 3. Thesemiconductor electron device according to claim 2, wherein said secondbuffer layer has a bandgap energy greater than a bandgap energy of saidfirst buffer layer and has an Al composition not less than 0.5 and athickness not less than 1 nm and nor greater than 10 nm.
 4. Thesemiconductor electron device according to claim 2, wherein said firstand second buffer layers comprise one of Mg, Be, Zn, and C in an amountof not less than 1×10¹⁶ cm⁻³ and not greater than 1×10²¹ cm⁻³.
 5. Thesemiconductor electron device according to claim 2, having an operatingcurrent of not less than 1 ampere or an operating voltage of not lessthan 100 volts.
 6. The semiconductor electron device according to claim1, wherein said buffer layer structure includes a plurality of saidfirst buffer layers and a plurality of said second buffer layers, whichare alternately laid one on another.
 7. The semiconductor electrondevice according to claim 6, wherein each of said first buffer layershas a thickness of not less than 0.5 nm and not greater than 20 nm, andeach of said second buffer layers has a thickness of not less than 0.5nm and not greater than 20 nm.
 8. The semiconductor electron deviceaccording to claim 6, wherein each of said second buffer layers has abandgap energy greater than a bandgap energy of each of said firstbuffer layer and has an Al composition not less than 0.5 and a thicknessnot less than 1 nm and nor greater than 10 nm.
 9. The semiconductorelectron device according to claim 6, wherein each of said first andsecond buffer layers comprises one of Mg, Be, Zn and C in an amount ofnot less than 1×10¹⁶ cm⁻³ and not greater than 1×10²¹ cm⁻³.
 10. Thesemiconductor electron device according to claim 6, having an operatingcurrent of not less than 1 ampere or an operating voltage of not lessthan 100 volts.